Low-molecular-weight polytetrafluoroethylene species have so far been used, for example, as additives to ink or cosmetic compositions or like other base materials for the purpose of reducing friction and improving slip characteristics on the base material surface or as additives to coating materials for the purpose of improving the surface texture of coatings.
Known as low-molecular-weight polytetrafluoroethylene species are those obtained by emulsion polymerization (cf. e.g. Japanese Kokai Publication S51-41085 and Japanese Kokai Publication H07-165828), those obtained by thermal degradation of high-molecular-weight polytetrafluoroethylene species (cf. e.g. Japanese Kokai Publication S49-39642, Japanese Kokoku Publication (Patent Publication) H07-5744, Japanese Kokoku Publication S50-15506, Japanese Kokai Publication S61-118331 and Japanese Kokai Publication S61-162503), and those obtained by subjecting moldings from high-molecular-weight polytetrafluoroethylene in powder, scrap or other form to irradiation treatment (cf. e.g. Japanese Kokoku Publication S52-25419, Japanese Kokoku Publication S49-48671, Japanese Kohyo Publication 2001-513529 and U.S. Pat. No. 3,766,031).
However, low-molecular-weight polytetrafluoroethylene powders obtained by any of those methods have problems, namely they tend to blow up as fine dust on the occasion of addition to other materials or, when they are electrostatically charged, they readily adhere to the hopper; thus, they are very poor in handleability.
That low-molecular-weight polytetrafluoroethylene powders are composed of very fine particles is also presumable as one of the causes of the problems of powder blowing up and adhesion to hoppers. If the size of the very fine particles is increased in an attempt to solve the above problems, however, there arises another problem, namely the dispersibility thereof on the occasion of admixing with other materials becomes poor.
There are no low-molecular-weight polytetrafluoroethylene powders available in the art that have both the characteristics, good handleability without powders blowing up or adhering to hoppers and good dispersibility as additives.
From the process simplification and molecular weight distribution narrowing viewpoint, those low-molecular-weight polytetrafluoroethylene species which can be directly obtained by polymerization and preferably need no post-treatment step for molecular weight reduction are preferred. As a method by which they can be obtained directly by polymerization, there is known the emulsion polymerization method (cf. e.g. Japanese Kokai Publication H07-165828) but any method involving suspension polymerization is not known.
Further, a powder with a specific surface area of 7 to 20 m2/g is known as a low-molecular-weight polytetrafluoroethylene species obtained by polymerization (cf. e.g. Japanese Kokai Publication H10-147617). This powder, however, is disadvantageous in that its particles are readily blown up or adhere to hoppers.
Furthermore, recent study results, among others, have revealed a suspected risk of perfluorooctanoic acid [PFOA] to the environment, and the United States Environmental Protection Agency (EPA) announced, on Apr. 14, 2003, that more intensive scientific researches concerning PFOA should be made (cf. e.g. EPA Report “Preliminary Risk Assessment of the Developmental Toxicity Associated with Exposure to Perfluorooctanoic Acid and Its Salts”, Internet <URL:http://www.epa.gov/opptintr/pfoa/pfoara.pfd>).